Are Galaxies Playing Catch with Black Holes?

Do black holes jump ship and wander off to other galaxies? If so, a galaxy called NGC 1277 may harbor a fugitive in its core. In 2012 astronomers discovered a supermassive black hole at its center with the mass of 17 billion suns—the most massive known. Normally, a black hole this enormous would be found in a much larger galaxy, which points to something unusual in NGC 1277's past. Two astronomers have one idea: What if the black hole was captured after being spit out of a galactic collision billions of years ago?

In fact, the black hole may be a reject from an even larger nearby galaxy. Billions of years ago two galaxies—each carrying a black hole in its core—slammed together to form a massive galaxy called NGC 1275. During the collision, the central black holes spiraled together, merged and recoiled into intergalactic space. The newly coalesced homeless black hole wandered the Perseus galaxy cluster until NGC 1277 passed close enough to gravitationally ensnare it. "It is speculative, but it's a fun story," says Gregory Shields, an astronomer at The University of Texas at Austin and lead author on a paper in The Astrophysical Journal Letters proposing this scenario. "You don't need to invent any new physics. You just need to have the luck to run into the smaller galaxy."

Computer simulations show that when two black holes merge, the uneven radiation of gravitational energy gives the resultant black hole a kick. In the case of supermassive black holes found in galactic centers that kick can launch the merged black hole at up to 5,000 kilometers per second—fast enough to leave its home galaxy.

Inspired by these simulations, Shields started working with Erin Bonning, an astrophysicist at Quest University Canada, to search for orphaned black holes. "We looked at the possibility that when a black hole is ejected like this, that it might drag a long disk of gas with it and continue to feed off that gas even while it was flying out of the original galaxy." The ensemble of black hole and gas would form a free-floating quasar: a brilliant engine of radiation driven by superheated gas spiraling around a massive black hole.

Although they have yet to turn up any quasars meandering between galaxies, the idea never went away. "It's such a fascinating process, you just have to keep thinking about it," Shields says. When the discovery of an oversize black hole in NGC 1277 was announced in 2012, Shields took note. "When I read that article, it just kind of popped to mind that [the black hole] formed in a bigger galaxy and got kicked out."

Karl Gebhardt, another U.T. Austin astrophysicist and co-discoverer of NGC 1277's black hole, is a bit skeptical: "It's a really interesting idea...but it's going to require a lot of luck." For Shields's scenario to work, three things need to happen: the black holes merge, kick out of another galaxy (NGC 1275) and then get caught by NGC 1277. Each of these events has a low probability just on its own. But in a big universe even unlikely things happen from time to time. "This galaxy is weird," Gebhardt says, "so the fact that a possible explanation is weird as well may not be such a surprise."

Figuring out just how weird will require looking at lots of other galaxies. "If there's no other galaxy that has as massive a black hole," Gebhardt explains, "then something with a very low probability could be a valid explanation." If, however, it turns out that oversize black holes aren't that unusual, then something else is going on. NGC 1277 may have once been a larger galaxy and had many of its stars and much of its gas ripped off in a near collision. Or maybe the black hole was shot out of a large galaxy and dragged that galaxy's nucleus along with it. Every scenario that astronomers can think of, though, starts with the black hole originating in a much bigger galaxy.

Figuring out where gargantuan black holes come from may lead to clues about how galaxies evolve. Astronomers have known for awhile that supermassive black holes and their host galaxies influence one another. As galaxies build through successive collisions, the black holes grow. A massive black hole can light up as a quasar by sucking in gas that otherwise would have gone into forming new stars. The gas then shoots out along jets thousands of light-years long, cutting off the galaxy's star formation. "People are seeing the symbiosis of a black hole, the energy it puts out as a quasar, and the ongoing evolution of the galaxy itself as a partnership that can have significant influences back and forth," Shields says. "We want to understand, then, how black holes come to be in galaxies."